1. Field of the Invention
The present invention relates to a multiple lamp balance transformer and a drive circuit, and more particularly to a transformer and a drive circuit used in a lamp equalizing circuit of a cold cathode fluorescent lamp (CCFL).
2. Description of Related Art
A cold cathode fluorescent lamp (CCFL) is generally used as a light source of a backlight module for liquid crystal display (LCD) panels. The CCFL is driven by a drive circuit of an inverter. Due to technological advancements and consumer requirements, the size of an LCD panel becomes increasingly larger and a single lamp no longer satisfies the illumination requirements thereof. Therefore, two or more lamps are needed. To assure an even brightness for the LCD panels, it is necessary to adjust the current of each lamp constantly, so that the current passing through each lamp is equal. Since the height of the CCFL varies and the CCFL has negative impedance, it is difficult to maintain consistent impedance for all lamps. As a result, the impedance of each lamp varies and the current cannot be equalized. If the currents between the lamps are not equal, the brightness will be uneven. An excessively large current will shorten the life of the lamps, and will result in a different aging rate of each lamp.
Reference is made to FIG. 1, which is a circuit diagram for using two independent balance transformers to balance the current of four CCFLs. In FIG. 1, a controller 10 supplies currents I1, I2, I3, I4 with a voltage required by a lamps L1, L2, L3, L4 through a boost transformer Tr to emit light. By adopting the impedance of the coil of a balance transformer as well as following Lenz's law, the operating currents passing through the lamps and the coils are balanced. If the currents I1, I2, I3, I4 are equal, the currents passing through each coil of the balance transformers T1, T2 are equal. Thus, the magnetic motive forces produced separately by the currents I1, I2 at the coil of the balance transformer T1 and the currents I3, I4 at the coil of the balance transformer T2 are equal and these magnetic motive forces cancel each other. As a result, there will be no magnetic flux between the balance transformers T1, T2. The leakage flux produced in each of the balance transformers T1, T2 passes through an air gap on the external side to complete a circuit. Since the magnetic resistance of the air gap is very high, the inductance effect caused by this circuit is negligible.
Further, a voltage detect/protect circuit 20 picks up the operating currents I1, I2, I3, I4 of the lamps and converts these operating currents I1, I2, I3, I4 into a voltage to be sent to the controller 10. The controller will then control and adjust the power output and supply the required voltage to the operating currents I1, I2, I3, I4 for emitting light currents according to an error signal.
However, only the currents I1, I2 or the currents I3, I4 of the prior art can be balanced independently, and the quantity of balance transformers used is increased according to the increase of the quantity of lamps. From FIG. 1, if it is necessary to drive four lamps, then two balance transformers T1, T2 are needed. Such arrangement not only increases the size of the whole drive circuit, but also increases the cost thereof, and wastes manpower and working hours for assembly.